Optical clocks represent the hallmark of precise timekeeping. The most precise atomic clocks are laboratory-based ones that utilize transitions in the microwave/optical domain of neutral atoms or trapped ions. These clocks define an international time scales, confirm the consistency of physical constants and stimulate the search for new physics outside the Standard Model. The other area is the regime of portable atomic clocks based on warm atomic vapor, which is small, power-efficient and deployable in the field with a variety of civilian applications, including sensing, communication and navigation. Two-photon resonance and optical transition based field deployable clocks present a viable alternative that may outperform current commercial chip scale clocks by at least a factor of 10 in terms of both long-term and short-term stability. For portable clocks, two-photon transitions in alkaline earth metals are of special interest. One of the systematic inaccuracies in the optical reference is eliminated by using counter-propagating laser beams to create Doppler-free spectroscopy. At IIT Tirupati, we are engaged in developing the next generation of atomic vapor based portable frequency standard using two-photon transition in Rubidium (Rb) for atomic sensing and quantum positioning applications. This article discusses the physics behind the two-photon transition in Rubidium atoms suitable for a clock reference. Briefly, it discusses the experimental methodology for realizing an optical clock using warm rubidium vapor.
This thesis presents details on the design, construction, and characterization of a novel compact optical clock based on a two-photon transition in calcium. Optical atomic clocks have reached unprecedented accuracy and precision and have the potential to revolutionize metrology. However, many of the applications of optical clocks, such as the detection of dark matter and gravitational waves, deep space navigation, measurements of variations in fundamental constants, and precision geodetic measurements, require compact portable clocks. Building optical atomic clocks on a scale suitable for most field applications is an ongoing challenge in the precision metrology community. This work describes a clock that can be made smaller and simpler than most existing optical clocks, while still operating with state-of-the-art precision. The basis for these improvements is using a two-photon transition, which eliminates the need for sub-Doppler cooling and tight confinement of the atoms. The design for the calcium two-photon clock is given, and the construction of the clock components is described. Experimental results are presented for the search for the clock transition frequency, which has never before been precisely determined. A detailed uncertainty budget is given, showing that this clock should be able to achieve a fractional frequency uncertainty of
∼10−17. In addition, a novel technique of ``magic polarization'' is presented which can be implemented to mitigate probe laser light shifts, long thought to be a serious impediment to precision two-photon clocks. An experiment in which a two-photon clock based on rubidium was launched and operated on a stratospheric balloon and subsequently recovered is also described. This work represents an advancement in the development of compact portable optical clocks which will further push the boundaries of precision measurement and our understanding of the universe.
This thesis investigates three novel architectures for compact and portable optical atomic clocks. Optical atomic clocks are known for their high accuracy and are some of the most precise instruments on Earth. Many applications of these instruments, such as space-based gravitational wave detection or tests of relativity, deep space navigation, or geodesy, require compact and portable systems. Creating compact clocks for field use, however, remains challenging as the equipment used to tightly control the motion and temperature of clock atoms is complex and adds to the size and weight of the system. The first two clock architectures described herein use two-photon transitions, with one system based on neutral rubidium (Rb) atoms and the other on neutral calcium (Ca) atoms. The third system uses samarium ions (Sm
2+) trapped in a crystal. Whereas using the two-photon transition in Rb and Ca eliminates the need for confinement of the atoms and for sub-Doppler cooling, the Sm
2+ concept embeds the clock atoms in an inherently rigid lattice. The Rb system was designed as a completely portable clock with a fractional frequency stability of 1 part in 10
13, which was compatible with a demonstration on a stratospheric balloon campaign. During the balloon campaign, the frequency standard was only able to reach a fractional frequency stability of $\delta\nu/\nu_0=6.4\times10^{-9}\persqrttau$ over the range 1-10000 s. The Ca and \sm systems target orders of magnitude higher precisions (greater than 1 part in 10
17 and 1 part in 10
20, respectively). The thesis reviews each system's design, details the experiments used to search for clock transition frequencies, and presents the results of these experiments along with other key measurements. Ultimately, this work provides new insights into the feasibility of these portable optical clock architectures.